Nuclear reactors are presently being designed, constructed, and operated in which the nuclear fuel is contained in fuel elements which have various geometric shapes, such as plates, tubes, or rods. The fuel material is usually enclosed in a corrosion-resistant, nonreactive, heat-conductive container, or cladding. The elements are assembled together in a lattice at fixed distances from each other in a coolant flow channel or region forming a fuel assembly, and sufficient fuel assemblies are combined to form the nuclear fission chain reacting assembly or reactor core capable of a self-sustained fission reaction. The core, in turn, is enclosed within a reactor vessel through which a coolant is passed.
The cladding serves several purposes, and two primary purposes are: first, to prevent contact and chemical reaction between the nuclear fuel and the coolant or the moderator, if a moderator is present, or both, if both the coolant and the moderator are present; and second, to prevent the radioactive fission products, some of which are gases, from being released from the fuel into the coolant or the moderator, or both, if the coolant and the moderator are present. Common cladding materials are stainless steel, aluminum and its alloys, zirconium and its alloys, niobium, certain magnesium alloys, and others. The failure of the cladding, i.e., a loss of the leak-tightness, can contaminate the coolant or moderator and the associated systems with radioactive, long-lived products to a degree which interferes with the plant operation.
The important requirements for materials used in nuclear reactor construction include low absorption for thermal neutrons, corrosion resistance, high ductility and mechanical strength. Zirconium-based alloys sufficiently satisfy these requirements and they are widely used for such purposes, "Zircaloy-2" and "Zircaloy-4" being two of the important commercial alloys commonly finding such use. These alloys exhibit corrosion under normal boiling water reactor operating conditions, resulting in the spalling of thick oxides from channels and the thickening of oxides on fuel rods. The spalling of oxide flakes leads, in some instances, to development of high-radiation fields in the vicinity of control rod mechanisms where the flakes collect; and the presence of thick oxide layers reduces heat-transfer efficiency and can result in local overheating of fuel cladding.
There is a desire to improve corrosion resistance of zirconium-based alloys to high-temperature water and steam without sacrificing other properties of the tubes made from such alloys. Differences in corrosive resistance between the inner circumference and outer circumference of a cladding tube can be provided by way of composition gradients. For example, corrosion resistance at one surface can be enhanced by plating or otherwise forming a composite structure. Such techniques can be costly and it is desirable to form a tube of uniform composition with improved corrosion resistance.